The present disclosure relates to an electrophotography type image forming apparatus, and more particularly to an image forming apparatus using a two-component developer including carrier and toner, and adapted to develop an electrostatic latent image by holding only the toner on a developing roller.
There are known a single-component developing method and a two-component developing method, as a developing method for an electrophotography type image forming apparatus. In the single-component developing method using a developer only composed of toner, a magnetic brush composed of carrier and toner is not formed. Accordingly, there is no likelihood that an electrostatic latent image on a photosensitive drum may be disturbed by the magnetic brush. Thus, the single-component developing method is suitable for high quality image formation, as compared with the two-component developing method.
The single-component developing method, however, has a difficulty in stably maintaining a charging amount of toner for a long period, as compared with the two-component developing method. Further, in the case where a color toner is used, the toner is required to have a light transmittance. In view of this, the toner is required to be non-magnetic toner, which makes it further difficult to stably maintain a charging amount of toner. Therefore, in most of the cases, the two-component developing method using carrier as a medium for charging and conveying toner is employed for a full-color image forming apparatus.
The two-component developing method is suitable for a longtime use of toner, because a stable charging amount is secured for a long period. However, the two-component developing method is not advantageous in the aspect of image quality, as compared with the single-component developing method, because the aforementioned magnetic brush adversely affects the image formation.
In recent years, there has been noticed a touchdown developing method having the advantages of both of the single-component developing method and the two-component developing method. In the touchdown developing method, a magnetic brush is formed on the surface of a magnetic roller by a two-component developer containing toner and carrier, and a thin layer of toner is formed on the surface of a developing roller by conveying only the toner from the magnetic brush. Then, the toner is caused to fly from the thin layer of toner onto the surface of a photosensitive drum where an electrostatic latent image is formed for developing the electrostatic latent image into a toner image. The touchdown developing method is advantageous in securing a longtime use of toner by applying the two-component developing method to a toner charging region, and in securing a high-quality image by applying the single-component developing method to a developing region.
The touchdown developing method, however, involves the following problem. A thin layer of toner is formed on the developing roller by using a magnetic brush and application of a bias voltage. As the bias voltage in one direction of electric field is continued to be applied to the developing roller, toner is adhered to the developing roller. Further, there occurs a developing ghost (hysteresis) phenomenon that a ghost image after fly of toner from the developing roller onto the photosensitive drum appears at a second turn of the photosensitive drum. To prevent such a phenomenon, it is necessary to perform a so-called refreshing operation of peeling off the thin layer of toner that has been formed on the developing roller, and forming another thin layer of toner on the developing roller. As the method for performing the refreshing operation, there are known a first method of peeling off a thin layer of toner by e.g. a blade or an elastic roller, and a second method of peeling off a thin layer of toner by applying a bias voltage in an interval between an image formation for a preceding sheet and an image formation for a succeeding sheet to generate a reverse potential difference between the developing roller and the magnetic roller.
Further, if a condition that the printing rate of an image to be formed is low is continued for a certain period, the amount of toner to be conveyed from the developing roller onto the photosensitive drum decreases. Deterioration of toner resulting from a longtime holding of the toner on the developing roller may cause various image-related problems such as image density lowering or a fogged image. In a conventional image forming apparatus, the image printing rate is calculated, and if the calculated image printing rate is lower than a predetermined rate, toner is forcibly consumed in an interval between image formations for preceding and succeeding sheets, in other words, toner on the developing roller is caused to forcibly fly from the developing roller onto an electrostatic latent image formed on the photosensitive drum, for conveying toner from the magnetic roller onto the developing roller by the amount of toner that has been caused to forcibly fly onto the electrostatic latent image. Thus, a thin layer of toner is constantly formed by supply of fresh toner from the magnetic roller.
Conventionally, a peeling off operation of peeling off a thin layer of toner has been performed after completion of a forcible consuming operation of forcibly consuming toner for stably performing the peeling off operation and the forcible consuming operation between image formations for preceding and succeeding sheets. It is necessary to set the interval between image formations for preceding and succeeding sheets long for performing the peeling-off operation and the forcible consuming operation. This necessitates raising the system speed to meet a processing speed required by the apparatus specifications. Raising the processing speed, however, results in a problem such as toner fly.
In view of the above, an object of the present disclosure is to efficiently perform a peeling off operation and a forcible consuming operation in a touchdown developing method, with a reduced interval between image formations for preceding and succeeding sheets.